1. Field of the Invention
The present invention relates generally to optical fibers, and more specifically to microstructured optical fibers, methods for locating discontinuities in microstructured optical fibers, methods for monitoring the draw of microstructured optical fibers, and methods for coupling microstructured optical fibers.
2. Technical Background
Optical fibers formed completely from glass materials have been in commercial use for more than two decades. Although such optical fibers have represented a leap forward in the field of telecommunications, work on alternative optical fiber designs continues. One promising type of alternative optical fiber is a microstructured optical fiber, which includes holes or voids running longitudinally along the fiber axis. The holes generally contain air or an inert gas, but may also contain other materials.
Microstructured optical fibers may be designed to have a wide variety of properties, and may be used in a wide variety of applications. For example, microstructured optical fibers having a solid glass core and a plurality of holes disposed in the cladding region around the core have been constructed. The arrangement, spacings and sizes of the holes may be designed to yield microstructured optical fibers with dispersions ranging anywhere from large negative values to large positive values. Such fibers may be useful, for example, in dispersion compensation. Solid-core microstructured optical fibers may also be designed to be single mode over a wide range of wavelengths. Solid-core microstructured optical fibers generally guide light by a total internal reflection mechanism; the low index of the holes can be thought of as lowering the effective refractive index of the cladding region in which they are disposed.
One especially interesting type of microstructured optical fiber is the photonic band gap fiber. Photonic band gap fibers guide light by a mechanism that is fundamentally different from the total internal reflection mechanism. Photonic band gap fibers have a photonic band gap structure formed in the cladding of the fiber. The photonic band gap structure may be, for example, a periodic array of holes having a spacing on the order of the wavelength of light to be propagated in the fiber. The photonic band gap structure has a range of frequencies and propagation constants, known as the band gap, for which light will not propagate in the photonic band gap structure. The core of the fiber is formed by a defect in the photonic band gap structure cladding. For example, the defect may be a hole of a substantially different size and/or shape than the holes of the photonic band gap structure. Alternatively, the defect may be a solid structure embedded within the photonic band gap structure. Light introduced into the core will have a propagation constant determined by the frequency of the light and the structure of the core. Light introduced into the core of the fiber having a frequency and propagation constant within the band gap of the photonic crystal structure will not propagate in the photonic band gap cladding, and will therefore be confined to the core. A photonic band gap fiber may have a core that is formed from a hole larger than those of the surrounding photonic band gap structure; in such a hollow-core fiber, the light may be guided substantially in a gaseous medium, lowering losses due to absorption and Rayleigh scattering of glass materials.
The propagation of light in a hollow core photonic band gap fiber is strongly dependent on the hole size, pitch, and symmetry of the photonic band gap structure. A slight change in the photonic band gap structure may perturb its properties such that a light propagating in the core is no longer forbidden to propagate in the photonic band gap structure, destroying the light-guiding properties of the fiber. In fabrication of a photonic band gap fiber, it is necessary to draw the fiber with a high degree of precision, so that the desired band gap properties are achieved.
It is difficult to perform diagnostic measurements on hollow core photonic band gap fibers. If a fiber does not guide light, it may be due to a break in the fiber, or due to a shift of the band gap caused by a deviance from the desired fiber structure. The band gap may be shifted to a wavelength outside the measurement range (e.g. by an incorrect scaling of the photonic band gap structure during the drawing of the fiber). Alternatively, the band gap may be destroyed by disorder in the photonic band gap structure. If the fiber does not guide light, standard methods of measuring fiber properties cannot be used.
Microstructured optical fibers (both photonic band gap fibers and index-guided fibers) may support guided modes that are not circularly symmetric. In one conventional type of solid core microstructured optical fiber, a solid core region is surrounded by a plurality of holes formed in a cladding material. The holes nearest the core region may be disposed, for example, in a regular hexagon around the core region. The core of this conventional microstructured fiber supports a guided mode having a somewhat hexagonal shape (i.e., having a C6 rotation axis). When splicing or otherwise coupling together two lengths of such fibers together, it is not only necessary to align the cores positionally; it is also necessary to rotationally align the fibers so that the propagation modes of the two fibers are in substantial rotational alignment.